Electroplating is a widely used process by which a layer of conductive material, typically a metal, is deposited onto at least one surface of a substrate. In many applications, such as in the fabrication of multi-layered printed circuit boards, it is desirable to evenly deposit the material onto the substrate surface(s) such that it will have a uniform thickness.
Electroplating involves immersing a substrate (i.e., circuit board) into an electrolytic bath between conductive anode walls (i.e., sidewalls of an anode "basket" structure) contained within a tank. The bath typically includes a conductive solution. Conductive material to be deposited on the substrate, such as nickel or copper nuggets, are located in close proximity of the anode walls.
During the process, a positive voltage is applied to the anode walls and a negative voltage is applied to the substrate (which acts as a cathode). In the fabrication of printed circuit boards, a layer of conductive material (from which traces may be etched through a photoresist process) typically is formed on the board, before the board undergoes electroplating. This conductive layer enables the printed circuit board to serve as a cathode.
When power is provided, a voltage drop is created between each anode and the substrate cathode. Positive ions (positively charged molecules) of the to-be-deposited material (i.e., copper, in the case of a printed circuit board) are formed within the solution. The ions are attracted to the cathode and repelled from the anodes, causing movement of the ions from the anodes to the cathode substrate through the solution. In this manner, the material is deposited onto exposed surfaces of the substrate.
In the fabrication of a circuit board, "through holes" are formed within (through) the board before the electroplating process. It is desired during electroplating that the conductive material not only be evenly deposited on the surfaces of the board, but also on the exposed surfaces within the through holes. Thus, boards typically are moved back and forth toward and away from each anode within the bath solution to cause the solution, and contained copper ions, to flow through the holes. It is particularly important in the electroplating of circuit boards that the deposited copper layer attain a uniform thickness.
A number of natural phenomena occur in the electroplating process which cause the material to be deposited unevenly on the substrate. For one, each ion has an associated weight. Thus, gravity causes the ions to fall within the tank as they migrate from the anodes to the cathode substrate. In addition, charge build-up tends to occur at edges of the cathode substrate causing a greater concentration of material deposition to occur in these areas. The fact that some substrates (which have different lengths) may not extend in length to the bottom of the tank further increases the concentration of material deposition occurring at the bottom edges of such substrates.
Much effort has been devoted towards minimizing the effects of such phenomena and factors so that even deposition is achieved. One common approach includes placing physical barriers, called "shields", between the anodes and cathode substrate adjacent substrate areas (such as the bottom edge, for example) where the deposition concentration tends to be higher. The shields offer a high resistance path to the material ions from anodes to cathode. Other approaches, typically used in conjunction with the shield approach, include (1) "sparging", which involves injecting bubbles (i.e., of air) into the solution in the immediate vicinity of the substrate cathode and, (2) physically agitating (setting in motion) the bath solution, both of which approaches tend to cause a more even material deposition.
Another approach involves reducing the voltage applied across the anodes and cathode, which voltage reduction tends to yield a more uniform deposition. The voltage reduction approach, however, also significantly slows down the process, particularly when the reduction is substantial, rending such approach infeasible in high volume circuit board manufacturing, where minimizing throughput time is of critical importance.
One prior art shielding approach is that described in U.S. Pat. No. 4,879,007 to Wong, titled Shield for Plating Bath, assigned to Process Automation International Limited, issued on Nov. 7, 1989 (hereinafter "the PAIL" patent). The PAIL patent approach includes forming holes within sidewalls of a floating panel positioner, which positioner retains the bottom edge of each circuit board substrate within an electroplating bath substrate and moves the retained circuit boards back and forth (toward and away from each anode wall). The sidewalls offer a high-resistance path to the migrating ions as they physically can pass only through the holes or must migrate around (i.e., over) the sidewalls. The sidewalls are spaced slightly from (located in close proximity of), and extend upwardly only a short distance from the bottom edge of, each circuit board. The sidewalls therefore act to shield ions only from a bottom area of the circuit board.
While the PAIL patent device is generally effective at limiting the commonly increased deposition concentration at and near the bottom edges of circuit boards, the approach suffers from a number of drawbacks. With sparging, bubbles can get trapped in small-pitched circuit board traces, which trapped bubbles block the flow of material ions, causing an uneven material deposition in such areas. In the PAIL patent approach, due to the close proximity between the shields and circuit board, the shield intersects and breaks up some sparging bubbles. In addition, the shield tends to move the bubbles toward and into contact with the circuit board, resulting in an increased number of bubbles that get trapped.
Another drawback of the PAIL patent approach includes what is referred to as the "trilobe" effect. When ions pass through each shield hole, they tend to spread out in a conical fashion due to their electrical attraction to the cathode circuit board. The overlap of ions emanating from any three adjacent shield holes causes a greater material deposition concentration in the overlapping areas. The result is referred to as the "trilobe effect" and is shown in FIG. 1. Shown is material 10 deposited on a substrate from ions passing through three adjacent shield holes. The deposited material forms three overlapping round areas 12, 14 and 16, each round area being formed from ions passing through a different one of the three adjacent holes. In the illustrative example shown, an area of overlap 18 has an increased material deposition concentration.
In an electroplating process, the area of overlap between adjacent holes, if any, depends primarily on the spacing of the holes, the distance between the shield and the circuit board, and the magnitude of voltage applied, as well as other factors. If overlapping occurs, then a greater concentration of deposited material occurs in the areas of overlap, as shown. If overlapping does not occur leaving spaces between the round deposited areas 12, 14 and 16, then areas will be left with a lesser concentration of deposited material. As a result, the material is not uniformly deposited. The fixed distance between the circuit board and the shields in the PAIL patent approach only exacerbates the problem because areas of overlap remain in the same locations, causing a continuous build-up in such areas during the electroplating process.
One further significant drawback of the PAIL patent device, and of other prior art shielding approaches, results from the fact that the shields are fixed in size and location with respect to the circuit board. Circuit boards, by contrast, vary in size. As a result, shielding which may be ideal for one board of a particular size is less than satisfactory for a board of a different size.
A general object of the present invention is to provide an improved electroplating shielding method and device which avoids the drawbacks of the prior art.